Paul, I think that dropping onto the knee is actually a faster smoother movement if you perform it properly. If you don't drop to a knee (MMA), 1. chances are that you are not low enough 2. you are gonna telegraph your movement 3. you're gonna take a few punches to the face for not dropping your level. 4. If you look down, you have a high possibility of getting kneed than if you did a wrestling shot. If you don't drop to a knee (grappling), 1. you telegraph your movement, 2. your center of gravity is high and you will get countered by a good fighter, 3. your balance is hindered.
Dropping to "one knee" is the correct way to shoot. The reason I put quotations around it is because when you "drop to one knee", that knee should only be on the ground for a split second as you explode forward. It gives you the proper level, doesn't jeopardize your balance, gives you a focused forward explosion, and allows for ankle picks and lifts.